Seismic wave amplifier



; 1942- H. HOOVER, JR

- SEISMIC. WAVE AMPLIFIER Filed July 26, 1 939 Macaw: can van/e5- Pas/rm: Cuemrulas Cou r1201. VOLTAGE nvmvrox. HEesEerfioovaeJe.

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ATTORNEYS.

Patented Dec. 1, 1942 SEISMIC AMPLIFIER Herbert Hoover, Jr., Sierra Madre, Califi, assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application July 26, 1939, Serial No. 286,648

11 Claims. (01. 179-171) My invention pertains to seismic wave amplifiers using automatic amplitude control (AAC). The main object of my invention is to provide stability in an amplifier using automatic ampli-.

tude control.

Another object is to produce stable distortionless operation of an AAO operated amplifier throughout a wide range of amplification.

Another object is to produce stability of an AAC operated amplifier by applying gain control voltages to a plurality of grids of differing characteristics in the main amplifier.

Another object is to proportion the AAC voltage between two grids of a vacuum tube of an amplifier so as to produce stable operation of the amplifier.

To accomplish these objects, I utilize an amplifier including two gain control elements having gain vs. control voltage characteristics of opposite curvature, and provide the amplifier with an automatic amplitude control means comprising means for deriving two automatic amplitude control voltages from the amplifier output, and

means for applying these automatic amplitude control voltages to said gain control elements in such a manner that both control voltages simultaneously change the amplifier gain in the same direction. Throughout this specification gain is used to mean the ratio of the amplifier output signal to the amplifier input signal.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the in-' vention herein described, as various formsmay be adapted within the scope of the claims.

Referring to the drawing:

Fig. l is a schematic diagram. of a preferred embodiment of my invention.

Fig. 2 is a graph illustrating gain characteristics obtainable in amplifiers using automatic amplitude control.

In Fig. 1, waves from a receptor signal source I are applied through an input transformer 3 to an amplifier shown in dotted lines 2. Signals impressed on the grid 8 of tube 4 produce currents at the anode l6 which in turn cause signals to pass through filter B and to be impressed on grid 9. Signals appearing at the anode II) are passed through the filter l to the grid ll of tube Filters 6 and l are designed so as to limit the pass band of the amplifier in any desired manner.

One part of the current flowing through anode I3 is applied to the recorder or other reproducer I4 and another part of the current is applied to the AAC circuit 20, through condenser 22 and resistor 23. Battery 26 preferably biases the tube to cut-off. By using triode 25, the signal impressed on the grid 24 is amplified as well as rectified, and a large control voltage appears across the resistor 21 connected between anode 2,8 and ground 2|.

This control voltage appearing across resistor 21 is applied to suppressor grid 15 of tube 4 through the low pass filter consisting of resistor 30 and condenser 3|. A portion of the control voltage is applied to control grid .8 of tube 4 through resistor 32 and the secondary of transformer 3. Resistors 21, 30 and 32 and the condensers 29, 3|, and 33 cooperate to reduce A. C components of the governing voltage before application to the gain control grids 8 and IE or the amplifier.

To understand the operation of my invention reference may be had to Fig. 2 in which ordinates represent gain and abscissa represent control voltage.

age to signal grid 8 alone will tend to cause the amplifier to oscillate when no signals are being received, or when only those of small amplitude are impressed on the input terminals. Such oscillation occurs because small signals or even tube noise appearing at the grid of tube 25 will cause a relatively large change of gain in the main amplifier channel. Since the control grid of tube 4 is affected, a corresponding surge of current will appear in the plate circuit of the-tube 4. This surge will be amplified by the rest of the amplifier, finally appearing at the grid of tube -25 again, and the process will be repeated. Various manifestations of instability or oscillation may be present, depending on many factors entering into the design of any particular amplifier, but'clepending especially on the various constants of filter elements 21, 29, 30, 31, 32, 33, 6, l, 22, or 23.

Thus for a low control voltage such as that at point A instability is likely to occur, as the gain is high under these circumstances and varies rapidly with small fluctuations of control voltage. While stable operation is more likely at a point A" the gradual slope of the characteristic curve in this region requires that very large voltages be supplied to grid I by the AAC circuit 20 in order to limit the output to a sufllciently narrow range. For these reasons, characteristic is not satisfactory for limitation of the output signal strength over a wide range of input signals.

If, however, the control voltages are applied to suppressor I! alone, the gain characteristic obtained will be somewhat as illustrated by curve b. Operation with such a characteristic curve is more likely to be stable and the output limited to a narrower range than can be obtained by operation with a characteristic such as a. However, for large signal strengths corresponding to point B, instability may arise because of the steepness of the gain vs. control voltage characteristic in this region, and distortion may occur because of overload of the grid 8 by the impressed signal. Operation on curve b has the further disadvantage that at low signal strengths corresponding to point B", the gain is relatively insensitive to changes in control voltage. The steepness of curve a decreases and that of curve b increases with control voltage over a substantial voltage range. Curves a. and b have positive and negative curvature respectively. It will be noted that in the example given the curvature of curve b increases throughout a substantial range of control voltage, viz., from about 3 to about 18 volts, while the steepness of the slope increases over a range extending from about 3 volts to 22 volts. By positive and negative curvature characteristics a. and b I mean to bring out the fact that in the operating range the radius of curvature of curve a points upward from the control voltage axis, while the radius of curvature of curve b points downward. Expressed in other words, in the operating range, the second derivative of the gain with respect to the absolute value of control voltage is positive for curve a and negative for curve b.

In accordance with my invention I proportion the governing voltage between a grid I5 having a negative curvature characteristic and a gradual cut-oil. grid Shaving a positive curvature characteristic in such a manner that stable, substantially distortionless, amplification is obtained throughout the sensitivity range desired. By applying properly proportioned control voltages to both the signal grid 8 and suppressor i5, I am able to control the gain readily at both high and low signal amplitudes. I have found that if I apply all the control voltage to a suppressor i5 and about one fourth of the control voltage to the signal grid .8 of certain types of pentode vacuum tubes, I am able to accomplish the desired result. Such a ratio is suitable for tubes such as the 687 having a characteristic such as that illustrated in Fig. 2. Other ratios are suitable with other tubes.

My invention is particularly applicable to seismic prospecting in which stable automatic amplitude control over a large range of signal strength is necessary.

In practice, I may prefer to apply AAC voltages in the manner described to amplifier tubes in two or more stages of the amplifier 2 and thus limit the output to a narrower amplitude range and at the same time attain stable AAC action. a

I claim:

1. In an amplifier utilizing automatic amplitude control having an input and an output. gain control elements within said amplifier intermediate said input and output, means responsive to output signals for deriving corresponding amplitude control voltages, means for applying a part oi said amplitude control voltages to one of said gain control elements having a negative curvature characteristic in which said negative curvature increases with control voltage over a substantial control voltage range, and means for applying a part of said amplitude control voltage to one of said gain control elements having a. positive curvature characteristic, both parts of said control voltages tending to simultaneously affect the gain in the same direction, whereby stable automatic amplitude control action is attained over a wide range .of incoming signal strength.

2. In an amplifier having an input and an output, an automatic amplitude control circuit, gain control elements intermediate said input and said output, means for applying a part of signals appearing in said output to said automatic amplitude control circuit, means for deriving amplitude control voltages from said automatic amplitude control circuit, amplitude control means for applying amplitude control voltages to one of said gain control elements having a negative curvature characteristic in which said negative curvature increases with control voltage over a substantial voltage range, and amplitude control means for applying a portion of said amplitude control voltages to a gain control element having a positive curvaturecharacteristic, both parts of said control voltages tending to simultaneously affect the gain in the same direction, whereby stable automatic volume control action is attained over a wide range of incoming signal strength.

3. An amplifier comprising an input and an output, an automatic amplitude control circuit, a signal grid in an amplifying device'within said amplifier, said signal grid being capable of varyplitude control circuit, and means for applying related parts of said amplitude control voltages to said grids, both parts of said control voltages tending to simultaneously ailect the gain in the same direction, whereby stable automatic volume control action is attained over a wide range of incoming signal strength.

4. In an amplifier having an input and. an output, an automatic amplitude control circuit, a signal grid in an amplifying. device within said amplifier, said signal grid being capable of varying the gain ratio rapidly for small control voltages and slowly for large control voltages applied to said signal grid alone, an auxiliary gain control grid being capable of varying the gain ratio slowly for small control voltages and rapidly for large control voltages applied to said auxiliary grid alone, means for applying signals appearing insaid output portion to said automatic amplitude control circuit to produce amplitude control voltages, means for applyingsaid amplitude control voltage to said auxiliary grid, and means for applying a related portion of said amplitude control voltage to said signal grid, both parts of said control voltages tending to simultaneously affect the gain in the same direction, whereby stable automatic volume control action is attained over a wide range of incoming signal strength.

5. In an amplifier having an input and an output, an automatic amplitude control circuit, a signal grid in an amplifying device within said amplifier, said signal grid being capable of varying the gain ratio rapidly for small control voltages and slowly for large control voltages applied to said signal grid alone, an auxiliary gain control grid in said amplifying device said auxiliary grid beirrg capable of varying the gain ratio slowly for small voltages and rapidly for large voltages applied to said auxiliary grid alone, means for applying a part of signals appearing in said output portion to said automatic amplitude control circuit means for deriving i amplitude control voltages from said amplitude control circuit, means for applying a large portion of said amplitude control voltage to said auxiliary grid and means for applying a small portion of said amplitude control voltage to said signal grid, both parts of said control voltages tending to simultaneously affect the gain in the same direction, whereby stable automatic volume control action is obtained for either large or small signals passing through said amplifier.

6. In an amplifier having an input and output, a plurality of amplifying tubes connected in cascade intermediate said input and said output, a first gain control grid within one of said amplifying tubes having a. gain control characteristic of positive curvature in a first control voltage operating range, a second gain control grid within one of said amplifying tubes having a gain control characteristic of negative curvature in a second control voltage operating range, said negative curvature increasing with control voltage in at least a part of said second range, means connected to said output for deriving a unidirectional control voltage which varies as an increasing function of the output signal strength, means for separately applying related portions of said control voltage to each of said grids in said operating ranges to vary the gain of said amplifier inversely as the strength of the output signal strength.

7. In an amplifier having an input and output, a multl-grid amplifying tube connected in amplifying relation intermediate said input and said output, the signal grid of said tube having a vari-' able 1. gain control characteristic and a suppressor grid of said tube having a gain control characteristic such that the gain is relatively insensitive to voltage changes in a low gain control voltage range, and the gain decreases rapidly with increase of voltage in a high gain control voltage range, means connected to said output for derlving a pulsating unidirectional control voltage which varies as an increasing function of the output signalstrength, means for applying a portion of said control voltage to said signal grid to vary the gain of said amplifier inversely as a function of the output signal strength, and means for applying a portion of said control voltage to said suppressor grid also to vary the gain of said ampliner inversely as a function of the output signal strens h.

8. In an amplifier having an input and output,

at least one amplifying tube connected in ampliresonant characteristic connected in the anode circuit of said tube, means for applying waves passing through said filter to said output, means connected to said output for deriving a unidirectional control voltage which varies as an increas-, ing function of the output signal strength, means for applying a portion of said control voltage to said suppressor grid to vary the gain of said amplifier inversely as a function of output signal strength without substantially affecting the overall frequency response of said amplifier and means for applying a portion of said control voltage to said signal grid also to vary the gain of said amplifier inversely as a function of output signal strength and to maintain the negative potential on said signal grid in excess of the amplitude of signals impressed on said signal grid.

9. In an amplifier, an amplifier tube having a cathode, a control grid adjacent said cathode, said signal grid having a variable ycharacteristic, an anode, a suppressor grid adjacent said anode, a positive screen grid intermediatesaid signal grid and said suppressor grid, a simle rectifier having at least an anode and a cathode coupled to said output, a resistor and condenser connected in parallel between said rectifier anode and rectifier cathode, a first low pass filter connecting said rectifier anode and said suppressor grid, and asecond low pass filter connecting an intermediate point on said resistor with said control grid.

10. In an amplifier having an input and output, a first gain control element intermediate said input and said output having a gain control characteristic of positive curvature with a steep slope at relatively low voltages only in the range of control voltage, a. second gain control element intermediate said input and said output having a gain control characteristic of negative curvature which has a. steep slope at relatively high voltages only in the range of control voltages, means connected to said output for deriving a pulsating unidirectional control voltage which varies as an increasing function of signal strength, means for applying at least a portion of said control voltage to each of said control elements in such a manner that the effective amplifier gain in said regions of steep slope is less than it would be if a control voltage were applied to only one of said control elements, whereby stable automatic amplitude control action is obtained throughout the entire range of operation.

11. An amplifier as recited in claim 10 wherein said second gain control element has a characteristic with 9. flex point at a value of gain less than about one fifth the maximum gain of said characteristic.

' HERBERT HOOVER, Jn. 

